1. Field of the Invention
This invention relates to prevent oxide recess formation in a shallow trench isolation process, and particularly relates to a quarter micron or beyond process preventing oxide recess formation in a shallow trench isolation process.
2. Description of the Prior Art
In fabricating a transistor, as the scale of the transistor scaling down, the isolation technology progresses from local oxidation (LOCOS) to shallow trench isolation (STI). Especially when the line width shrinks from 0.35 .mu.m to 0.25 .mu.m or below, the shallow trench isolation is widely used in isolation technology. In fabricating a transistor, the traditional shallow trench isolation technology is described below. First, referring to FIG. 1, provide a substrate 100, then subsequently form a pad oxide layer 101 and a silicon nitride layer 102 on the substrate 100. Secondary, refer to FIG. 2, develop a photoresist pattern 103 on the silicon nitride layer 102 followed by etching the wafer. Thus the trench 104 is formed in the substrate 100, and the active area of the transistor is also defined. Then form a thermal oxide liner 105 in the trench 104, and the thermal oxide liner 105 is composed of silicon dioxide.
Next, remove the photoresist pattern 103 followed by depositing a silicon dioxide layer on the topography of the wafer by CVD (Chemical Vapor Deposition) and then planarize the CVD silicon dioxide layer by CMP (Chemical Mechanical Polish) method. Then, referring to FIG. 3, the trench isolation 106 is formed in the trench. Subsequently, remove the silicon nitride layer 102, and remove the pad oxide layer 101 by a first diluted HF (Hydrofluoric Acid). The first diluted HF is a mixture of hydrofluoric acid and ammonium fluoride at a ratio 1:10. The first diluted HF uses 15 seconds to remove the pad oxide layer 101. Meanwhile, referring to FIG. 4, the corner of the trench isolation 106 is attacked and the profile of the trench isolation 106 shrinks at the corner 107. Besides, the substrate 100 is exposed thereby. Refer to FIG. 5, the next step is to form a sacrifice oxide layer 108 on the substrate 100, and then use the ion implantation to form the channel in the active area of the transistor.
The next step is to remove the sacrifice oxide layer 108 by etching the wafer using a second diluted HF, referring to FIG. 6, thus the sacrifice oxide layer 108 is removed and the corner 105 of the trench isolation 106 is further attacked. The second diluted HF is a mixture of hydrofluoric acid and ammonium fluoride at a ratio 1:100. The second diluted HF uses 300 seconds to remove the sacrifice oxide layer 108. Thus the trench isolation 106 is fabricated in the substrate 100. To fabricate a transistor, the following step is to grow a gate oxide layer 110 on the substrate 100 and the trench isolation 106. Subsequently, deposit a gate poly layer 112 on the gate oxide layer 110. It is obvious that because the corner 105 (FIG. 5) of the trench isolation 106 shrank, referring to FIG. 7, the recess 120 is produced at the corner of the trench isolation 106.
In addition, if the shallow trench mentioned above is used in isolating the transistor, some traditional processes are applied to fabricate a transistor. After a series of process fabricating the transistor, the top view of the structure of the transistor mentioned above is shown in FIG. 8. The trench isolation 106 between the dashed rectangle and the solid rectangle adjoin to the drain and the source electrode, which also slightly overlapped with the gate poly. A cross sectional view of the transistor is cut by the broken line aa', and the right portion of the cross sectional view of the transistor near the terminal a' of broken line aa' is shown in FIG. 9.
As shown in FIG. 9, when the recess becomes serious, the kink effect becomes more serious. The kink effect, i.e. the local decrease in the threshold voltage at the channel edge, is always inherent to an abrupt isolation/active transition of STI process, which tighten the process window for manufacturing STI. So, in order to minimize the kink effect, it is important to reduce the recess 120.